Moldings based on plastics, which are to be used together with metal parts and which require a coating, are increasingly being employed in the automobile industry. Plastics moldings of this type consist of polypropylene or other thermoplastics or temperature setting plastics and can be coated with plain colors and effect colors with or without pretreatment. It is also possible for plastics parts not to be directly coated with lacquers but to be clad with lacquer-coated backing sheets. The coated backing sheets are applied to the motor vehicle parts by deformation with the application of heat and/or vacuum.
Processes of this type for the manufacture of interior parts of motor vehicles are already in use. When effect lacquer systems which are customary for motor vehicle bodywork are used in the outer colors, this results in visual defects in the uniformity of the effect coating and in cracks when it is subjected to dilatation, and these fissures continue in the sheet used, with tearing of the same. This can be circumvented by heating the coated sheet to high temperatures, where 145.degree. C. is quoted as the minimum temperature, whereby the lacquer film becomes sufficiently flowable at the time of deformation. These processes are described in EP-A-0 251 546 96B1 and EP-A-0 261 815 B1, wherein polyester or polyurethane sheets are used as the sheet, a commercially available solvent lacquer (Durethane 101 PPG Industries) is used as the lacquer, and a commercially available acrylic adhesive is used as the adhesive. It is disadvantageous that during the flow processes which are utilised the position of lamellar effect pigments in relation to each other and their geometric alignment within the effect lacquer coat alter in such a way that the uniform appearance of the effect lacquer coat is impaired, whilst no defects occur for solid colour lacquers. Moreover, on account of the high temperatures many polymers of interest cannot be used as sheet materials, since the optimum temperature range for forming processes, with the maximum elongation at tear for thermoplastics or the crystallite melting range for partially crystalline polymers, has already been exceeded. The high deformation temperatures also make the deformation process susceptible to defects at different temperatures within the production range.
EP-A-0 441 888 claims a process in which dual effect lacquer coats comprising effect pigments of different sizes are applied to polymer backing sheets, wherein these coats together produce effect coatings which are particularly attractive visually. Sheets for thermal deformation can be coated using this system if the melting temperature of the lamellar effect pigments is around 5.degree. C. to 50.degree. C. higher than the temperature of thermal deformation. Therefore, high deformation temperatures are necessary, or low-melting effect pigments have to be used. Moreover, two coats of lacquers containing effect pigments have to be applied.
One fundamental problem is that during the plastic flow processes which occur on deformation, the positioning of the effect pigments is altered in an irregular manner corresponding to the flow of the sheet material, and the uniform appearance of the deformed sheet is thus impaired and visual defects such as streak-like phenomena occur. However, even tests with elastomeric thin film substrates have shown that cracking and detachment of applied coatings occur during deformations, and thus effect-coated moldings cannot be produced.
EP-A-0 352 298 B1 describes a process in which fluoropolymers are used as binder vehicles for lacquers for coating sheets which are first thermally deformed into their final geometry and are subsequently back-filled, on their back face, with plastic molding material in an injection molding process. A disadvantage here is that two operations are necessary in order to deform the sheet and to anchor it on the supporting plastic material, and that fluoropolymers are expensive and problematical materials.